A Numerical Study of Dripping on the Ignitability of a Vertically Oriented Thermoplastic Material Locally Heated by an Irradiation Source

This study numerically investigates the effect of dripping on the ignitability of a vertically-oriented thermoplastic material subjected to localized radiant heating within a 2D domain. Thermoplastic material is modeled as a phase-change material with the prescribed solidification/melting temperature, and its rate of gasification (pyrolysis) process is described by the Arrhenius law. Molten matter can move downward due to the gravitational force, and accordingly, the gasification (pyrolyzed) surface area vary over time. Time-dependent heat and mass transport processes, including global one-step pyrolysis reaction are solved using FLUENT combined with appropriate user-definition functions (UDFs) developed by the authors. In order to simplify the problem, gas-phase kinetics were not considered, instead, the (expected) ignitability was estimated on the basis of the fuel mass flux evolved from the interface. The viscosity of the molten matter was varied as a numerical parameter in order to modify the degree of time-dependent deformation of the molten matter, and the influence of its dripping on ignitability is discussed. The numerical results clearly indicate that dripping occurs quickly when lower viscosity is imposed, although the trajectory of its mass-center yields a similar trend, irrespective of the viscosity (its moving speed greatly differs, though). As the dripping was pronounced, the thickness of the molten matter in the heated zone became thinner causing it to heat up quickly, at such point immediate gasification occurred, resulting in the ignition delay time becoming shorter. Interestingly, as the dripping was further promoted in a very-low viscosity case, the molten matter quickly flows-off prior to substantial gasification is occurring, resulting in the chance of ignition being inhibited. In this respect, dripping exhibited two competing effects on ignitability, implying that there were optimal conditions for the ignition with the shortest delay time. A simple strategy to mimic such a dripping effect in a conventional numerical model (without developing a dripping model) is also discussed.

Automated summary

This study investigates the effect of dripping on the ignitability of vertically-oriented thermoplastic material under localized radiant heating. The viscosity and drip speed of molten matter have an impact on ignition delay time.